1. Field of the Invention
The present invention relates to gamma correction, and more specifically to a digital gamma correction circuit and a digital gamma correction method, which are used in display devices such as liquid crystal displays (LCDs) and plasma display panels (PDPs).
2. Description of the Background Art
In recent years, it has often been the case that display devices (e.g., liquid crystal display devices) for displaying high-definition and high-quality images are used in electronic equipment such as cell phones and digital cameras. For such display devices, a digital gamma correction circuit for performing gamma correction on inputted digital image data is indispensable for displaying more natural images. Also, for electronic equipment, such as scanners and printers, which performs digital image processing, the digital gamma correction circuit is indispensable for outputting more natural images.
For the digital gamma correction circuit, there are conventionally known approaches for performing gamma correction by using a lookup table (hereinafter, referred to as the “LUT”) or region splitting and arithmetic processing.
Digital gamma correction circuits used in the LUT approach are provided with an LUT, composed of a ROM or such like, having stored therein values of output data (correction data) in association with values of input data. For example, digital gamma correction circuits for use in a liquid crystal display device are provided with an LUT having stored therein correction data for compensating for a gamma characteristic of a liquid crystal panel, in association with values of input data. The digital gamma correction circuits used in the LUT approach read correction data corresponding to values of input data from the LUT to perform gamma correction on the input data.
On the other hand, in the case of digital gamma correction circuits used in the region-splitting approach, the range of values that can be taken by input data is split into a plurality of regions, for each of which a linearly-represented gamma correction characteristic is defined. The digital gamma correction circuits used in the region-splitting approach perform gamma correction on input data by arithmetic processing in accordance with a gamma correction characteristic of a region including the value of the input data.
Regarding the digital gamma correction circuits used in the LUT approach, Japanese Laid-Open Patent Publication No. 8-51557 discloses that when reading correction data from an LUT composed of a ROM, a result of adding an offset value to a video signal is provided as an address. In addition, regarding the digital gamma correction circuits used in the region-splitting approach, Japanese Laid-Open Patent Publication No. 11-32237 discloses that a gamma characteristic curve is split into a plurality of regions, and the split gamma characteristic curve is approximated by straight lines connected at the borders of the regions, so that gamma correction is performed based on the approximation straight lines. Furthermore, regarding the digital gamma correction circuits used in the region-splitting approach, Japanese Laid-Open Patent Publication No. 11-120344 discloses that positions of break points of a gamma correction broken line are fixed in the direction of input data, so that slope data can be designated for each of a plurality of regions split at the fixed positions, and positions of the break points in the direction of output data can be calculated before performing gamma correction.
In the digital gamma correction circuits used in the LUT approach, however, the LUT is required to store correction data corresponding to all values of input data. Therefore, in the digital gamma correction circuits used in the LUT approach, the number of pieces of correction data that have to be stored tends to become significant as the number of bits in input data is increased.
Also, in order for the digital gamma correction circuits used in the region-splitting approach to become applicable to devices with various gamma characteristics, it is necessary to, for example, provide coordinates of the break points, and locate the borders of the regions in suitable positions in accordance with the gamma characteristics of the devices. In addition, in order to obtain substantially ideal smooth gamma correction characteristics, it is necessary to increase the number of regions. Therefore, in the digital gamma correction circuits used in the region-splitting approach, the number of pieces of data that are to be preset tends to be increased in order to obtain smooth gamma correction characteristics.